The present invention relates to injection molding machines and relates in particular to those machines which include a hot runner system composed of a plurality of conduits for serving a plurality of mold cavities with thermoplastic material where there is a conduit individual to each mold cavity.
The invention relates further to a so called shooting pot or metering device incorporated in a conduit for advancing precise quantities of thermoplastic material to each mold cavity, on a cavity by cavity basis.
Typical prior art shooting pots or metering devices are disclosed and described in U.S. Pat. Nos. 4,717,324, 4,775,308, 4,808,101 and 4,863,665, assigned to Husky Injection Molding Systems Inc., to which the present application is assigned.
To aid in establishing the setting or background of the present invention, U.S. Pat. No. 4,717,324 to Schad et al., is incorporated herein by this reference thereto.
In these prior art devices, plastic material is introduced into a shooting pot from a source under the control of a conduit or channel valve which is independent of the shooting pot, per se.
That is, the valve means in a given hot runner conduit is located between the plastic material source and the shooting pot upstream of the shooting pot.
Valve means of various types are employed to prevent backflow of the resin in the cylinder or shooting pot. Obviously, the channel used to fill the cylinder must be blocked when the cylinder's piston is moved forward to discharge or inject the resin into the subsequent downstream channel, nozzle, and ultimately, the mold cavity. This valving is typically achieved by either a check valve in the supply channel or a rotary or sliding valve actuated to block the supply channel.
These devices have inherent disadvantages. Check valves require some backflow of the resin to cause them to close. This reduces the amount of resin in the cylinder by some variable amount and consequently reduces the accuracy of the metering of the resin being injected into the mold. The known volume of the cylinder controlled by the back position of the piston during filling is compromised by this small backflow of resin to cause the check valve to operate.
Rotary and sliding valves invariable leak resin or seize up. It is very difficult to control the clearances in these moving assemblies to prevent leakage and avoid seizing. These components operate at elevated temperatures and thermal expansion causes them to change size, sometimes sufficiently to seize. Although this expansion can be predicted for a known temperature of operation, sometimes this expected temperature is not the one at which the device is ultimately operated. Also processing a different resin may require a different operating temperature and consequently a different thermal expansion of the valve components.
A further disadvantage of prior art valve mechanisms is that they occupy valuable space inside the hot runner system.